Frequency modulation method and device for high intensity discharge lamp

ABSTRACT

An electronic control gear for a HID lamp receives an input signal for operating the lamp and frequency modulates the input signal to generate a frequency modulated output signal that drives an arc of the lamp. The frequency modulation of the output signal sweeps continuously between a low frequency modulation and a high frequency modulation, the low frequency modulation being a frequency f 1  in a range of 125 kHz to 150 kHz and the high frequency modulation being a frequency f 2  in a range of 230 kHz to 300 kHz, where f 2− f 1  is at least 0.4*f 1 . A power amplifier changes an amplitude of the frequency modulated output signal during the low frequency modulations so that a current amplitude of the frequency modulated output signal is substantially constant, and a variable gain transformer adjusts a voltage of the frequency modulated output signal during starting of the lamp.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of, and claimspriority from, U.S. application Ser. No. 11/665,002, now U.S. Pat. No.7,944,151, with a §371 filing date of Oct. 29, 2008, which is the U.S.National Stage Application of PCT Application No. PCT/US05/33478,entitled FREQUENCY MODULATION METHOD AND DEVICE FOR HIGH INTENSITYDISCHARGE LAMP and filed on Sep. 20, 2005, the entire contents of all ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention is directed to a method and device that frequencymodulates an input signal that operates a high intensity discharge (HID)lamp to thereby generate a frequency modulated output signal that drivesan arc of the lamp.

An arc of a HID lamp may be driven with a high frequency signal; thatis, a signal whose current is generally sinusoidal or triangular and hasa frequency in the range of about 100 kHz to 300 kHz with no, or nearlyno, DC component in the current. A high frequency signal for driving anHID lamp is desirable because such signals may be generated withcomponents that are generally smaller and cheaper than those thatgenerate lower frequency signals. However, high frequency signals areaccompanied by acoustic resonance phenomena that appear strongly in HIDlamps.

Frequency modulation of the signal that drives the arc of the lamp is aknown technique for at least partially avoiding acoustic resonance whenan HID lamp is driven with a high frequency signal. Modulation methodsand devices tend to be complicated and costly, especially whenattempting to develop a device that is useful in various HID lamps.Acoustic resonance frequencies vary from lamp to lamp and within a lampdepending on lamp temperature.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a noveldevice and method for frequency modulation of a signal that drives anarc of a HID lamp and avoids the problems of the prior art.

A further object of the present invention is to provide a novel deviceand method in which the frequency modulation of the signal sweepscontinuously between a low frequency modulation and a high frequencymodulation, and more particularly where the low frequency modulation isa frequency f1 in a range of 125 kHz to 150 kHz and the high frequencymodulation is a frequency f2 in a range of 230 kHz to 300 kHz, wheref2−f1 is at least 0.4*f1.

A yet further object of the present invention is to provide a novelelectronic control gear (ECG) for an HID lamp and method of operatingthe HID lamp in which the frequency modulation of the signal sweepscontinuously between a low frequency modulation and a high frequencymodulation, a power amplifier changes an amplitude of the frequencymodulated signal during the low frequency modulations so that a currentamplitude of the frequency modulated signal is substantially constant,and a variable gain transformer adjusts a voltage of the frequencymodulated signal during starting of the lamp.

These and other objects and advantages of the invention will be apparentto those of skill in the art of the present invention afterconsideration of the following drawings and description of preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C show frequency versus time plots for various frequencymodulation sweeps of the present invention.

FIGS. 2A-B show current and voltage versus time for an embodiment of thepresent invention and FIG. 2C is a combination of FIGS. 2A-B.

FIG. 3 is a plot of a frequency spectrum of an embodiment of the presentinvention.

FIGS. 4-5 are plots of SPD ratio versus frequency in tests of thepresent invention.

FIG. 6 is a schematic representation of an embodiment of a device of thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The method and device of the present invention spread a frequencyspectrum of the lamp current over a wide range of frequencies so as toachieve a better, and desirably nearly equal, distribution offrequencies over the wide range. By so doing, the method and devicedecrease a magnitude of the frequency spectrum to a level low enough toavoid exciting acoustic resonance. For example, acoustic resonance at afrequency is avoided when a spectral content at that frequency is lessthan about 1%, when measured as a ratio of spectral power distribution(SPD; also known as power spectral density—PSD).

The method and device frequency modulate an input signal, such as theinput signal 4 shown in FIG. 6, that operates the lamp (shown as theload in FIG. 6) to generate a frequency modulated output signal thatdrives the arc of the lamp and whose frequency modulation sweepscontinuously between a low frequency modulation and a high frequencymodulation. By sweeping the frequency of modulation between twoparticular modulation frequencies, the magnitude of the frequencyspectrum between those particular modulation frequencies is reduced to alevel low enough to avoid exciting acoustic resonance.

In one embodiment, the low frequency modulation is a frequency f1 in arange of about 125 kHz to 150 kHz and the high frequency modulation is afrequency f2 in a range of about 230 kHz to 300 kHz, where f2 minus f1is at least 0.4 times f1. The magnitude of the frequency spectrumbetween f1 and f2 is low enough to avoid exciting acoustic resonance.

A preferred technique for decreasing a magnitude of the frequencyspectrum to a level low enough to avoid exciting acoustic resonance isto hold the current amplitude constant, or nearly so, and to sweep thefrequency of the frequency modulation more or less linearly from thefrequency of the low frequency modulation to the frequency of the highfrequency modulation. This provides a saw tooth or triangular patternsuch as shown in FIGS. 1A-C. As shown therein, the pattern may besymmetric or asymmetric. The sweep may deviate slightly from linear toprovide a more constant current amplitude so as to achieve a more nearlyconstant output power from the device.

The high and low frequency modulation frequencies should be selected toavoid a visible flicker in the arc. To this end, the low frequencymodulation is a frequency f1 in a range of about 125 kHz to 150 kHz andthe high frequency modulation is a frequency f2 in a range of about 230kHz to 300 kHz. The sweep time (time from the low frequency modulationto the high frequency modulation and back to the low frequencymodulation) is about 2-20 ms.

More particularly, f1 may be one of 125 kHz, 130 kHz, and 135 kHz, f2may be one of 230 kHz, 240 kHz, and 250 kHz, and the sweep time may beone of 8.33 ms and 10 ms. Alternatively, f1 may be 100 kHz, f2 may beone of 140 kHz and 150 kHz, and the sweep time may be one of 8.33 ms and10 ms. In tests, lamps swept with a lower frequency modulation at 100kHz or below or above 150 kHz (and 250 kHz high frequency modulation)had arcs that did not appear to be as stable (the arc moved around inthe burner) as those of lamps that were swept with a lower frequencymodulation in the range of 125 kHz to 150 kHz. The frequency of the highfrequency modulation produced satisfactory results in the range of 230kHz to 300 kHz (frequencies higher than 300 kHz were not tested). Sweeptimes were also investigated. Slow sweeps caused a visual oscillation inthe light amplitude perhaps due to a small frequency dependent variationin burner power, however there did not appear to be an associated arcinstability. Long sweep times, such as 30 ms produced an annoyingflicker, while sweep times from 2 ms to 15 ms did not produce a visualarc disturbance.

The lamps investigated were one sample of each of the following: PhilipsCDM-R 70 W/830 PAR 30, Sylvania MPD70 PAR 30, and OSRAM Powerball HCI-T70 W/WDL. FIGS. 2A and 2B show the current and voltage waveforms,respectively, at the end of a sweep during the rapid (about 0.06 ms)return from 250 kHz back to 125 kHz (e.g., as in FIG. 1A). Thesewaveforms were acquired for the Powerball lamp. The upward portion ofthe linear frequency ramp took 10 ms. The current and voltage areessentially identical sine waves that lie right on top of eachother—that is to say: identical except for a scaling factor of 100 V/Aas seen by comparing the left and right scales in FIG. 2C.

The signal from a photodiode, which was illuminated by an OSRAMPowerball HCI-T 70 W/WDL, lamp, was analyzed by a Fourier transformsignal analyzer. FIG. 3 shows the resulting spectrum wherein the lamppower is mostly at twice the frequency of the lamp voltage, orcurrent—in this case, sweeping from 250 kHz to 500 kHz. A small signalappears in the 125 kHz to 250 kHz range owing to asymmetries in theplasma. The slope of the signal is due to thermal inertia in the plasmaand capacitance in the photodiode. The lamp was powered by HF swept from125 kHz to 250 kHz in 10 ms. The arc appeared stable under theseconditions. This plot shows an average over 200 acquired spectra.

FIG. 4 shows the SPD ratio measurement for the 125 kHz to 250 kHz scan.Note the flat plateau at a level of about 1.3% from 250 kHz to 500 kHzshowing the power frequency components at double the current or voltagefrequencies. Note also the presence of a 0.4% level from 125 kHz to 250kHz owing to asymmetries in the plasma as described above. Clearly, thewide scan technique is doing a good job of reducing the spectral contentat any one frequency down to nearly 1%. This level has been shown to bea reasonable level to avoid acoustic interference.

FIG. 5 shows the SPD ratio for the 100 kHz to 250 kHz scan which inducedinstabilities in the two PAR 30 lamps. The plateau is again at twice thecurrent or voltage frequencies as in FIG. 4—here spanning the range from200 kHz to 500 kHz at a level of about 1.3%. Apparently, there is aresonance in the 200 kHz to 250 kHz region that is especially sensitiveto excitation.

FIG. 6 shows an embodiment of a device 10 for sweeping HF power in anHID lamp. The capacitor 16 coupling between the function generator 12and the power amplifier 14 serves to diminish the amplitude at the lowfrequency end of the sweep to compensate for a drop in amplifier andcoupling gain at the high frequency end. The result, as can be seen inFIGS. 2A-C, is fairly constant power supplied to the lamp versusfrequency. The variable gain matching transformer 18 provides highvoltage during starting and reasonable impedance matching to theamplifier output when the lamp warms up. The transformer 18 has multipleoutput taps that are selected by a switch that insures a continuouscurrent conduction path when it is being switched to avoid extinguishingthe arc. The turns ratio may be set to 4 for starting and deceased toabout 2.1 for operating. Similarly, the function generator 12 used tocontrol the sweep frequencies and amplitudes produces continuous outputwhile making-adjustments, unlike some more modern devices. Anexperimental setup included an IEC.TM. F34 function generator and anENI.TM. 1140LA power amplifier.

In another embodiment, an amplitude of the frequency modulated outputsignal is changed during at least one of the high and low frequencymodulations, preferably during the low frequency modulation, so that acurrent amplitude of the frequency modulated output signal issubstantially constant. The power amplifier 14 provides this function. Avoltage of the frequency modulated output signal may be adjusted duringstarting of the lamp, for example by the variable gain transformer 18.

While embodiments of the present invention have been described in theforegoing specification and drawings, it is to be understood that thepresent invention is defined by the following claims when read in lightof the specification and drawings.

1. An electronic control gear (ECG) for a high intensity discharge (HID)lamp, comprising: a function generator that receives an input signal,wherein the input signal operates the lamp, wherein the functiongenerator frequency modulates the input signal to generate a frequencymodulated output signal that drives an arc of the lamp, wherein thefrequency modulation of the input signal by the function generatorsweeps continuously between a low frequency modulation and a highfrequency modulation, wherein the low frequency modulation has amodulation frequency f1 and the high frequency modulation has amodulation frequency f2, where f2 minus f1 is at least 0.4 times f1, andwherein the low frequency modulation is in a range of about 125 kHz to150 kHz and the high frequency modulation is in a range of about 230 kHzto 300 kHz; a power amplifier that receives the frequency modulatedoutput signal from said function generator and changes an amplitude ofthe frequency modulated output signal during at least one of the highand low frequency modulations so that a current amplitude of thefrequency modulated output signal is substantially constant; and avariable gain transformer that receives the frequency modulated outputsignal from said power amplifier and adjusts a voltage of this signalduring starting of the lamp.
 2. The ECG of claim 1, wherein a sweep timefrom the low frequency modulation to the high frequency modulation andback to the low frequency modulation is about 2-20 ms.
 3. The ECG ofclaim 1, wherein a frequency of the frequency modulation changessubstantially linearly between the low frequency modulation and the highfrequency modulation so that the frequency of the frequency modulationhas a generally triangular pattern.
 4. The ECG of claim 3, wherein thetriangular pattern is symmetric.
 5. The ECG of claim 3, wherein thetriangular pattern is asymmetric.